positive local sharpening values add the local amount to the global for net effect.
whereas negative values mask the global effect in a non-additive fashion, i.e.
sharpening at -50 will mask all sharpening regardless of what the global amount is set at. -1 to -49 partially mask global sharpening, and -51 and greater negative values will result in actual blurring.

I've found local "sharpening" at exactly -50 to be a surprisingly good substitute for local lum. NR - completely unsharpened noise is far less offensive to me. I voted for this FR/Idea, I'm just sayin': in the mean time...

Great idea ProDigiPhoto! The adjustment brushes are one area that afford a great opportunity for new features. I'm sure as more users let the team know about what they'd like to see available through the brushes we'll see capabilities expand.

positive local sharpening values add the local amount to the global for net effect.
whereas negative values mask the global effect in a non-additive fashion, i.e.
sharpening at -50 will mask all sharpening regardless of what the global amount is set at. -1 to -49 partially mask global sharpening, and -51 and greater negative values will result in actual blurring.

I've found local "sharpening" at exactly -50 to be a surprisingly good substitute for local lum. NR - completely unsharpened noise is far less offensive to me. I voted for this FR/Idea, I'm just sayin': in the mean time...

Local sharpening at -50 masks 100% of global sharpening effect.
-1 to -49: masks 2% to 98%.
(beware: local sharpening at -51 to -100 blurs)
(and for completeness: local sharpening at 1 to 100 adds the local amount to the global amount for net effect).

And of course there's the sharpening masking slider (set globally, but like all global sharpening sliders - it effects local sharpening too).

I like the idea of having a noise reduction mask (with a slider like the sharpening mask) better than the idea of a noise reduction brush. The reason being that all of Lightroom is non-destructive, so all the painting ends up bloating the xmp file (or the dng file if that's how you do it) ... which is OK, except that every time the image is rendered Lightroom has to do a load of processing (have you ever tried removing LOTS of dust from an image and seen how Lightroom then struggles?). The sharpening mask seems to be processed much more efficiently so I would expect a noise reduction mask to be the same.

Photoshop is there for more selective noise reduction.

(But I guess some photographers never use Photoshop and do everything in Lightroom - so they want everything in Lightroom. And of course just because a feature is there doesn't mean you have to use it).

This is, of course, a great idea. But more broadly, ALL the edit features in LR should be available as local edits when using the brush. Including NR, Tone Curve, HSL adjustments, etc. About the only things that would probably be impractical to implement at the distortion tools.

I'd go further than that and say that in addition to all modifiers being available as local edits (an extension of what we already have), there should also be a local edit mask for every modifier (completely new).

Interesting idea. As it stands, Lightroom allows negative values for all the locals it does support, which means its possible to "mask" any of those global adjustments. So would the distinction here be that instead of applying a negative number additively, this new modifier "mask" would apply a negative percentage instead of a negative offset? Or is the distinction just a UI thing? Or an implementation thing? (bitmap vs. "strokes").

Perhaps Adobe knows exactly what you are driving at and why, but some clarification would help some forumers (at least one anyway) to know what the rationale is for a new modifier mask vs local masking as implemented.

I mean my initial reaction to this idea is negative - introducing another way to accomplish the same thing would be like changing horses in mid-stream... - but maybe I just don't understand the advantages of your idea.

Effectively Lightroom does use an edit mask - but instead of painting on the mask as in Photoshop, you paint directly on the image. You can see the mask by hovering the brush over the adjustment point. So this is a user interface preference really. However it would be nice to be able to click on the adjustment point (after hovering on it to show the mask) and then to be able to edit the mask visually. (As a workaround this can be done now by sliding the exposure all the way to the right, for example, as this effectively shows the mask in white).

You can see the mask as you're applying the brush. Just check the "Show Selected Mask Overlay" in the task bar under the image after you've clicked on the brush tool. It will show the mask in pink as you apply the brush.

@RC - the problem with using opposite brush adjustment values to counteract a global modifier value is that as soon as you tweak the value of the global modifier you have to manually tweak the brush setting again to counteract it, as there is no link, they are completely independent.

Whereas, if every global modifier also had the option of a brush-applied mask (rather like a layer mask bound to an adjustment layer in photoshop) then you could quickly make (and continue to progressively tweak & refine) global changes to the whole image while excluding certain masked parts of the image from those adjustments.

This is in some ways equivalent to achieving the same via the existing brush tool, but that (currently) only features a subset of the full range of possible adjustments, and clearly it's also easier (and more efficient for LR) to paint a mask on the thing you want to exclude (as per my suggestion) rather than to paint everything else (as one would have to do at present).

More adjustments would be great of course - but I would like to see better ways of applying them too, especially using gradients, as these are much more efficiently implemented in Lightroom and also easier to use than a brush.

So more gradients (as in Photoshop) would be very useful. The ability to apply them to a rectangular selection would be lovely.

Yes of course that would be nice - but I was thinking of the processing overhead in Lightroom and also ease of use and not changing the user interface too much.

Another option that might not be too heavy on resources would be to be able to drag and skew the gradient bars (so drag them into a rectangular area and then skew them to cover more or less of the area).

I guess this comes down to where Lightroom ends and Photoshop begins ... and perhaps one discriminator is performance. For example, LR is very efficient size-wise with all the adjustments to a raw file taking only a few kilobytes or megabytes. Bring the same file into Photoshop and the file swells to hundreds of megabytes ... but now complex masks, for example, are processed very fast. This is important for me because I hold tens of thousands of images in Lightroom as developed raw files and only a few thousand that have been finished off in Photoshop. If adding features in Lightroom meant that I would have all the power of Photoshop that would be great ... but not if it came with a huge penalty in file size or processing requirement.

This really came to light for me when I took a batch of photos at f22 with a dirty sensor and the visible dust on the images was massive: I removed all the dust in one image in LR then synchronised all the other images. I then found that rendering the images was so slow that I got rid of the dust removal in Lightroom and did it in Photoshop instead. (BTW - my PC has 16GB memory, an i7-2600K processor, SSDs and performance disks, etc., so it's a fast machine by today's standards and yet it struggled with the dust removal).

So when it comes to voting for additional features in Lightroom I will favor features that do most of what I want but continue to do it efficiently rather than features that do everything that I want but do so at a high performance or file-size hit.

If I was a photographer who did all of his processing in Lightroom then my voting would be different :)